That might come as news to the record labels who have pursued legal action against similar locker services like MP3tunes.com over the years.

Google (like Amazon with its cloud service) maintains that Google Music is a backup service for users' own music collections -- no different than using a backup hard drive.

But over the years, some labels and publishers have maintained that any time a user streams a song over the Internet, royalty payments apply.

Amazon has had confrontations with the music labels over its cloud service, and EMI has been in a longstanding lawsuit against MP3tunes.com.

The service is focused on caching, not redownloading from the cloud, which could help on a legal basis. (Users can force a particular song to be cached locally so they can play it offline, but it's not permanent.) Part of the appeal of MP3tunes.com is that it can act as a full backup service. If you lose your collection, you can redownload it to your device. It appears that Google Music won't offer that -- although you'll be able to stream to any device, which amounts to the same thing for users.

Another drawback: because each song has to be uploaded to the server manually, uploading a collection could take days. One early tester said it's taken more than two days to download about 1,000 songs. If Google had agreements with the labels, it could simply let users upload a menu of the songs on their PC, then Google could deliver the songs directly from its servers.

Rosenberg also said that Google wanted to launch a music store at the same time as the Google Music service, but the company got hung up in negotiations with a couple of major labels -- the company wouldn't say which ones. Rather than launching a store only with content from independent labels and musicians, Google decided to launch the locker now, and wait until it can get the major label deals before launching a full store.

If Google wants a store, it'll need the cooperation of the recording industry.